1. Field of the Invention
The present invention relates to the processing of bitstreams encoded according to the MPEG standard, and more particularly to the format conversion of MPEG bitstreams.
2. Description of the Related Art
The MPEG (Moving Pictures Experts Group) standard proposes a set of algorithms dedicated to the compression of sequences of digital (audio/video) signals. The techniques used are based on the reduction in spatial and temporal redundancy of the sequence.
According to the MPEG standard, reduction in spatial redundancy is obtained by independently compressing the individual images, using a discrete cosine transform (DCT), quantization, and Huffman coding.
Reduction in temporal redundancy is obtained by exploiting the correlation that exists between successive and/or temporally close images in the sequence. Approximately, it is assumed that each portion of an image could be expressed locally as the translation of a portion of a previous and/or subsequent image in the sequence.
The foregoing corresponds to criteria that are well known to the prior art and hence are such as not to require further illustration herein.
In the actual use of the MPEG standard it is possible to transmit (or record) films, or in general video sequences, on a variety of channels and media, each of which has its own characteristics of capacity, speed and cost.
The transmission of a film (or of a video program in general) starting from the master recording may be done, for instance, on DVD media, via satellite, radio antenna, cable, and—to an ever-increasing extent—via telematic networks, such as Internet, and/or to mobile terminals, such as UMTS mobile terminals.
In order to make available a wide range of the aforementioned transmission modalities and to expand it even further, it is necessary to be able to adapt the characteristics of the MPEG bitstream to the transmission/storage channel, as well as to the characteristics of the display units employed to display the corresponding images.
The foregoing entails, among other things, the need to convert the format of an MPEG video bitstream in such a way that the resulting sequence is reduced, for example, to one quarter of the original format.
In order to obtain the above result, it is possible, for example, to apply the “canonical” solution represented in FIG. 1.
In the case of FIG. 1, an input MPEG video bitstream, designated by IS, is entered into a decoder 100, which is able to read and decode the bitstream IS with a format corresponding to a number of pixels Hor×Vert per image.
The signal thus decoded is fed to a set of downsampling filters 120 which are able to downsample the image starting from the original format Hor×Vert to a reduced format (Hor/N)×(Vert/M).
The signal thus subjected to conversion of format is entered into an encoder 140, which is able to generate a syntax in compliance with the MPEG standard, in view of its being sent on to a transmission/storage channel C and received in a decoder 160.
The decoder 160 is able to read and decode an MPEG bitstream with a number of pixels (Hor/N)×(Vert/M) per image, hence one with a reduced format, generating at output a corresponding video sequence OS.
The above technique in effect entails a conversion of the original MPEG bitstream (irrespective of whether this is in compliance with the MPEG2 specification or with the MPEG4 specification) into dec sequence of frames. The change of resolution is based upon a filtering action—this being preferably a finite-impulse-response (FIR) filtering—which is able to bring about a conversion based upon the availability of a certain number of pixels for each component of luminance and chrominance of the image. The pixels in question are multiplied by appropriate weights, and the results are accumulated and divided by the sum of said weights.
Certain pixels are not transmitted in the resulting image, depending upon the factor of change of resolution chosen. For this reason, starting from a bitstream decoded with an arbitrary bitrate B1, it is always possible to obtain a bitstream of bitrate B2 by simply connecting the output of the decoder to the input of the change-of-resolution block. The output of the latter is then connected to the input of the decoder, which can be programmed in such a way as to encode the signal at bitrate B2 and then transmit it to the decoder.
The above solution, which may be defined as an explicit transcoding, presents the fundamental drawback due to the computational complexity linked to its implementation, a computation complexity which renders the solution in question far from attractive in general and in any case unacceptable in those applicational contexts in which a sufficient processing capacity is not available.
A solution that is alternative to the canonical approach examined previously is described in the European patent applications Nos. 01830084.8 and 01830227.3, both in the name of the present applicant.
The above alternative solution is based upon a “transcoding” process which makes it possible to convert the format, i.e., change the resolution, by operating in the domain of the discrete cosine transform (DCT) of the prediction error. In this way, it is possible to reduce considerably the computational complexity of the operation of format conversion, thus reducing accordingly the implementational complexity.
In order to achieve a correct anti-aliasing filtering function, it is in any case necessary to resort to linear-filtering techniques, which require, as is known from the theory of digital filtering, the availability of a certain number of coefficients surrounding the ones that are being processed, the aim being to eliminate, or at least reduce as far as possible, edge effects.